Prothrombinase is composed of the protein cofactor, factor Va, and the enzyme, factor Xa, associated on a cell surface in the presence of divalent metal ions. Incorporation of factor Va into prothrombinase and its interaction with factor Xa results in a 300,000-fold acceleration of the catalytic efficiency of the enzyme as compared to the catalysis of the reaction by factor Xa alone. The procofactor, factor V, does not participate in prothrombinase. Following activation of factor V by thrombin, factor Va is composed of heavy and light chains associated via divalent metal ions. Both chains of the cofactor interact with factor Xa while only the heavy chain of the cofactor binds prothrombin. The factor Va cofactor activity is efficiently down-regulated following proteolysis of the heavy chain by activated protein C (APC) only in the presence of a membrane surface and results in the inability of the cofactor to bind factor Xa. Thus, the positive and negative regulatory processes associated with factor V activation and its inactivation are directly associated with the capability of the cofactor to be incorporated into prothrombinase and to bind factor Xa. The amino acids responsible for the interaction of factor Va with factor Xa and prothrombin remain to be identified. We have data demonstrating that the heavy chain of the cofactor possesses a binding region for factor Xa within amino acid region 323-331, whereas the NH2-terminal portion of the light chain (amino acid residues 1546-1558) also interacts with factor Xa. In addition, we have data suggesting that the COOH-terminal portion of the heavy chain contain an interactive site for prothrombin while previous data have suggested that a binding site for thrombin is located on the B region of the procofactor. The specific aims of this grant proposal are: (1) to identify and characterize the factor Xa-binding domain(s) on factor Va light chain; (2) to identify and characterize the thrombin and prothrombin-binding domain(s) on the factor V molecule; (3) to test the physiological relevance of our findings by studying the assembly and function of prothrombinase on platelets. To achieve these goals we have designed a series of experiments that are prioritized and integrated with complementary molecular and structural approaches. Characterization of the specific amino acid regions of factor V that are critical for its function will allow for a profound understanding of the macromolecular interactions that control prothrombinase and are required for its assembly, function, and specificity. We have established a system to study phospholipids-driven macromolecular complex formation, which may be a model for the generation of complexes that form extra-and intra-cellularly.